| Rule 1: | All non-zero digits and "trapped zeros" (zeros which occur between significant digits) are significant digits. |
| Rule 2: | Leading zeros are never significant digits. All they do is hold the decimal place. |
| Rule 3: | Trailing zeros to the right of the decimal are always significant digits. If they weren't no one would include them! |
| Rule 4: | Trailing zeros to the left of the decimal place may or may not be significant. It is ambiguous in these cases. To help remove the ambiguity, people often use scientific notation. Then if the zeros are significant digits, it is clear because the occur as trailing zeros to the right of the decimal place. |
| Rule 5: | Conversion factors which are based on definitions (such as 1000 mL per L or 3600 seconds per hour) have an infinite number of significant digits (because they are exact.) Exact numbers also occur in things you can count with zero uncertainty (such as 12 eggs or 37 railroad cars.) |
| Rule 6: | Some physical constants are so well determined that the number of significant digits can be chosen. An example of this is pi which can be used to 3 significant digits (3.14) or 8 (3.1415927) when really high prescision is required. |
Here are some examples.
| 12.34 g | 4 significant digits | Rule 1 |
| 0.000234 L | 3 significant digits | Rule 1, 2 |
| 102.0 m | 4 significant digits | Rule 1, 3 |
| 1200 km/sec | 2, 3 or 4 significant digits | Rule 1, 4 |
| 6.02 x 1023 atoms | 3 significant digits | Rule 1 |
| 1.400 x 10-3 mol | 4 significant digits | Rule 1, 3 |
| 0.002320 coulomb | 4 significant digits | Rule 1, 2 and 3 |
| 5280 ft/mile | infinite number of significant digits | Rule 5 |
| c (speed of light in a vaccum) | generally, as many as you need | Rule 6 |